xDSL is a generic term for various DSLs (digital subscriber line), and includes an ADSL (asymmetric digital subscriber line), an RADSL (rate adaptive digital subscriber line), a VDSL (V very high speed digital subscriber line), an SDSL (symmetric digital subscriber line), an IDSL (integrated services digital network-based digital subscriber line), an SHDSL (single-pair high-speed digital subscriber line), and the like. The xDSL is a high-speed data transmission technology for transmission on a phone twisted-pair cable. Apart from DSLs such as the IDSL and the SHDSL for baseband transmission, an xDSL for passband transmission enables, by using a frequency division multiplexing technology, the xDSL and a POTS (Plain Old Telephone Service, plain old telephone service) to coexist on a same pair of twisted-pair cables, where the xDSL occupies a high frequency band, the POTS occupies a baseband below 4 kHz, and a POTS signal is separated from an xDSL signal by using a splitter. A system providing access to multiple xDSLs is referred to as a DSL access multiplexer (Digital Subscriber Line Access Multiplexer, DSLAM for short). Due to a principle of electromagnetic induction, multiple channels of signals that are led to the DSLAM interfere with each other, where the interference is referred to as crosstalk (Crosstalk), as shown in FIG. 1 and FIG. 2. FIG. 1 shows a case of near-end crosstalk (NEXT), and FIG. 2 shows a case of far-end crosstalk (FEXT). Energy of both the near-end crosstalk (NEXT) and the far-end crosstalk (FEXT) enhances as a frequency band increases. xDSL uplink and downlink channels use the frequency division multiplexing, and therefore, system performance is not much damaged by the near-end crosstalk (NEXT). However, because a frequency band used for the xDSL becomes increasingly wide, the far-end crosstalk (FEXT) causes increasingly severe impact on transmission performance of a line.
To resolve the foregoing problem, at present, a Vectoring (crosstalk cancellation) technology is proposed in the industry, to cancel FEXT interference by using a signal processing method mainly according to a probability of joint receiving and sending at a DSLAM end, thereby finally cancelling FEXT interference in each channel of signal. FIG. 3 is a schematic principle diagram of synchronous sending and synchronous receiving at a DSLAM end. In an uplink direction, at the DSLAM end, FEXT information is extracted, by using an uplink crosstalk canceller, from a received signal that is sent by a client device, and is then removed from the received signal, to eliminate impact of FEXT, thereby implementing DSL performance in an ideal environment that is almost free from crosstalk. Likewise, in a downlink direction, the client device feeds back the FEXT information to the DSLAM end by means of agreement between an office terminal at the DSLAM end and a far-end terminal, and then, the DSLAM end precodes, by using a downlink vector pre-coder, these pieces of FEXT information into a normal signal to be sent. In this way, the precoded signal and FEXT cancel each other out during transmission, and the client device can receive correct information that is almost free from crosstalk.
In the prior art, when a new line is added in a system, a DSLAM end needs to initialize the new line, and in the initialization process, basic initialization and crosstalk cancellation initialization are alternately performed on the new line, that is, the DSLAM end performs basic initialization on the new line during a process in which an uplink crosstalk canceller and a downlink vector pre-coder of the system are updated again, so that the new line enters a data transmission stage (Showtime stage), and FEXT between the new line and an existing line at the Showtime stage is cancelled. Generally, both a central office device and a client device send pilot sequences on synchronization symbols (Sync Symbols), and a receive end feeds back an error to a VCE (Vectoring Control Entity, vectoring control entity) to calculate an uplink crosstalk canceller and a downlink vector pre-coder. If there are N lines in the system, N Sync Symbols are needed to completely estimate crosstalk between the N lines, thereby calculating the uplink crosstalk canceller and the downlink vector pre-coder for Vectoring. For the Walsh-Hadamard orthogonal pilot with an order of 2 raised to the nth power that is commonly used the industry at present, at least 2┌log 2N┐ Sync Symbols are needed. Generally, an interval between Sync Symbols in an xDSL is relatively large; for example, for VDSL2, two consecutive Sync Symbols are spaced by 256 symbols, about 64 ms. Therefore, when there are many lines, it needs to take a very long time to completely estimate an uplink crosstalk canceller and a downlink vector pre-coder for Vectoring for once. Especially during initialization, according to some adjustments in processing of a signal at an analog end or a digital end, multiple times of estimation are needed, which further prolongs a time for the initialization. In addition, it also takes a relatively long time for the new line to have a data transmission capability, which causes that a client device connected to the new line needs to wait a relatively long time to transmit data.